The present invention relates to pumps, and particularly to power-steering pumps for use in vehicle steering systems and controls therefore. Power-steering pumps for use in vehicle steering systems are well known and there are a plurality of power-steering pump constructions. Such pumps have associated therewith some means of controlling the flow of fluid to the steering system in response to a pressure demand for steering fluid. Further, normally some means of controlling the output flow from the pump is provided so that an excessive amount of flow from the pump is provided so that an excessive amount of flow at high vehicle speeds is not directed to the system.
Typically, and as shown in U.S. Pat. No. 3,200,752 the above-noted functions are provided by a bypass valve which responds to pressures acting thereon and bypasses output pump flow to the pump inlet. By controlling the output flow to the system by a bypass valve, the pump output is controlled and maintained so that a proper flow of fluid to the system in order to handle steering demand is provided and excessive flow to the system at high vehicle speeds is prevented. In the type of systems disclosed in U.S. Pat. No. 3,200,752 the bypass valve must bypass substantial volumes of fluid in order to provide for the proper operation of the pump in the system. For example, the amount of fluid bypassed at extremely high speeds may be in the order of 25 gallons per minute, which, of course, requires a substantially large valve for purposes of bypassing such flow.
Recently, as disclosed in U.S. Pat. No. 3,822,965, a substantial breakthrough in power-steering pumps was achieved. In U.S. Pat. No. 3,822,965, a bypass valve, as conventionally had been used in power-steering pumps, was eliminated. Rather than the use of a bypass valve for purposes of bypassing excessive flow of fluid from the pump, a pump construction and control was developed wherein the pump was unloaded due to movement of a cheek plate, which defined part of the pumping chamber in which the pump displacement structure operates. When the cheek plate moves, the pumping chambers connected with the inlet and the pumping chambers connected with the outlet are directly communicated to provide flow from the outlet to the inlet of the pump across the pump displacement structure and across a face of the cheek plate.
In U.S. Pat. No. 3,822,965, the cheek plate is moved due to fluid pressures acting thereon, and specifically a fluid pressure chamber is provided on the side of the cheek plate opposite the side facing the displacement mechanism. A control valve is provided in the system and which controls the pressure in the chamber or cavity. The control valve which controls the pressure in the cavity is a relatively small valve as compared to the bypass valve heretofore referred to and, of course, does not function as the main bypass for fluid, the main bypass being across the face of the cheek plate.